The invention relates to a reactive sputtering apparatus for etching or coating in which the discharge voltage of the electrode is a function of the quantity of reactive gas per unit volume, and the power supplied to the electrode can be switched between first and second values.
When coating substrates, for example substrates comprising glass, with metallic coating, for example with aluminum, sputtering devices are frequently used which comprise an electrode with a target. The target is sputtered due to the electrically charged particles impinging on it. In the process, the sputtered particles, potentially after a reaction with a gas, are deposited on the substrate. If the target is penetrated by a constant magnetic field, the combination of electrode, target and magnets, is referred to as a magnetron.
If the metal particles ejected from a target combine with a gas before their deposition on a substrate, the process is referred to as reactive sputtering.
If, for example, SiO2 is to be vapor-deposited onto a substrate, by means of Ar ions Si atoms are ejected from an Si target, which combine with oxygen which is introduced into the process chamber. The introduced oxygen at a constant electrical power affects the discharge voltage at the cathodes of the process chamber. If the discharge voltage is plotted at constant electrical power over the O2 concentration, a curve with hysteresis results (cf. T. Oyama, H. Ohsaki, Y. Ogata, S. Watanabe, J. Shimizu: xe2x80x9cHigh Rate Deposition of SiO2 Films by Pulse Sputteringxe2x80x9d, pp. 31-36, FIG. 3), Proc. of the 3rd International Sympositum on Sputtering and Plasma Processes, Jun. 8 and 9, 1995, Tokyo, Japan). With increasing oxygen concentration the discharge voltage initially decreases slightly and subsequently drops steeply to a low value. Starting at this value, the discharge voltage subsequently decreases only negligibly with increasing rising oxygen concentration. If the oxygen concentration is now reduced, the discharge voltage increases steeply starting at a specific concentration. Since the oxygen concentrations at which the voltage decreases, respectively increases, steeply are not identical, a hysteresis results.
Setting a stable operating point is made very difficult through this hysteresis because slight changes of the oxygen concentration, and/or the electrical power supplied, can certainly result in a jump of the discharge voltage.
The invention therefore addresses the problem of stabilizing rapidly and simply the operating point in the case of reactive sputtering.
This is done by selecting the power value so that at the same reactive gas flow rate, the target is sputtered in a metallic mode at the first power value and in an oxide mode at the second power value. The cathode is operated intentionally alternating between metallic mode and oxide mode, a mean value of the modes is formed so that the cathode can seemingly be stabilized to the intermediate metallic states between metallic and oxide. In the production of materials such as TiO2 this represents one way of being able to stabilize a high-rate operating points in spite of lacking electrical signals derived from the sputtering process.